1. Field of the Invention
The present invention generally relates to an in-line color cathode ray tube and, more particularly, to an exposure device used to make a luminescent phosphor deposited screen in an in-line color cathode ray tube.
2. Description of the Related Art
FIG. 1 of the accompanying drawings illustrates a schematic exploded view of a commerial in-line color cathode ray tube utilizing a finely perforated shadow mask that is currently available. The in-line color cathode ray tube illustrate therein includes highly evacuated envelope 13 having a funnel section 14 closed at a rear end thereof by a generally cylindrical neck section 15 and at a front end by a generally rectangular cup-shaped faceplate 16. The faceplate 16 has a generally rectangular inner surface area formed with a predetermined mosaic pattern of color emissive phosphor dots, corresponding to primary element colors (e.g., red, green and blue), to define a luminescent phosphor deposited screen 25 facing towards the interior of the evacuated envelope 13. The evacuated envelope 13 also includes a color selection electrode or a finely perforated shadow mask 17 having a multiple of apertures 18 defined therein in a predetermined pattern said perforated shadow mask 17 is supported in a position within the evacuated envelope 13 while spaced a predetermined distance inward from the luminescent phosphor deposited screen 25. The evacuated envelope 13 further includes an in-line electron gun assembly 19 held in a position within the neck section 15. The in-line electron gun assembly 19 includes three electron guns corresponding to primary element colors (e.g., red, green and blue) and arranged in line with each other and generally parallel to the scan direction of the electron beams emitted therefrom.
For deflecting the electron beams emitted from the in-line electron gun assembly 19 so as to permit the beams to scan the luminescent phosphor deposited screen 25 in a manner well known to those skilled in the art, a deflection yoke 20 having deflection coil assemblies is mounted on the exterior of the evacuated envelope 13 at a position generally aligned with the boundary between the funnel section 14 and the neck section 15.
In the conventional in-line color cathode ray tube utilizing the finely perforated shadow mask 17, the in-line electron gun assembly 19 produces the electron beams corresponding in number to the number of the electron guns, and the number of the primary colors, which electron beams subsequently travel through the fine apertures 18 in the perforated shadow mask 17. By projecting the electron beams through the finely perforated shadow mask 17, any single electron beam impinges upon the color emissive phosphor dots of a particular one of the primary colors. Image reproduction is accomplished by scanning the electron beams across the luminescent phosphor deposited screen.
The degree of coincidence in the geometric positional relationship between any single triad of the color emissive phosphor dots on the luminescent phosphor deposited screen 25 and any single electron beam which has passed through the associated aperture 18 in the finely perforated shadow mask 17 and subsequently impinges only upon such color emissive phosphor dots of a particular one of the primary colors is generally described in terms of the landing characteristic. The higher the degree of coincidence, the better the landing characteristic.
As hereinabove described, in the color cathode ray tube utilizing the finely perforated shadow mask, the magnetic fields generated by the deflection coil pair on the deflection yoke 20 in respective directions perpendicular to each other are utilized to cause the electron beams to deflect so as to scan across the luminescent phosphor deposited screen 25. On the other hand, the deposition of the color emissive phosphor dots on the screen area of the faceplate 16 to provide the luminescent phosphor deposited screen 25 is generally carried out by the use of an exposure system.
During the deposition of the color emissive phosphor dots on the screen area of the faceplate 16, attempts have been made to render the travel path of the light rays from an exposure light source to be aligned with the path of travel of any single electron beam, which may be depicted during the operation of the color cathode ray tube, as close as possible so that the landing characteristic can be favorably improved. An example of the conventional attempts is disclosed in, for example, the Japanese Laid-open Patent Publication No. 56-88231, published July 17, 1981. This prior art reference discloses the exposure system having an exposure light source of a type wherein the virtual center of the exposure light source in the horizontal direction (the widthwise direction of the luminescent phosphor deposited screen) is differentiated from that in the vertical direction. The principle of this prior art reference will now be discussed in detail with particular reference to FIGS. 2(a) and 2(b).
FIGS. 2(a) and 2(b) illustrate the exposure light source in the form of a generally cylindrical lamp (a high pressure mercury lamp) 1 in transverse and longitudinal sectional representations, respectively. The cylindrical lamp 1 reproduced therein includes a hollow cylindrical wall 11 made of quartz glass and a light emitting filament 12 extending over the length of the hollow cylindrical wall 11. Between the exposure light source 1 and the luminescent phosphor deposited screen of the faceplate 13 (FIG. 1), there is disposed a slitted member 2 positioned close to the exposure light source 1. The slitted member 2 is in the form of a light shielding plate having three slit-shaped light transmissive areas 21 defined therein while leaving light intercepting areas 22 around the slit-shaped light transmissive areas 21. As best shown in FIGS. 2(a) and 2(b), each of the slit-shaped light transmissive areas 21 is small in width as measured in an X-axis direction and long in length as measured in a Y-axis direction which is perpendicular to both of X-axis and Z-axis directions, but aligned with the vertical direction of the luminescent phosphor deposited screen 13 (FIG. 1).
With the exposure light source 1 and the slitted member 2 disposed as hereinabove described, the virtual position of the exposure light source as viewed from a point on the faceplate in the X-axis direction coincides with the position of the slitted member 2 while the virtual position of the exposure light source as viewed from a point on the faceplate in the Y-axis direction coincides with the actual position of the exposure light source 1, but not the position of the slitted member 2. In other words, the virtual position of the exposure light source in the X-axis direction is closer to the faceplate than the virtual position of the exposure light source on the Y-axis direction by a distance corresponding to the distance H between the slitted member 2 and the exposure light source 1. Accordingly, the relative position of the center of deflection induced by the horizontal and vertical deflection coils of the deflection yoke which produce the magnetic fields in the respective directions perpendicular to each other is favored, if the horizontal deflection coil is positioned on one side close to the faceplate, because the travel path of the light rays from the exposure light source 1 can be brought into alignment with the path of travel of the electron beams as close as possible.
With the use of the prior art exposure system outlined above, the inventor of the present invention has conducted a series of experiments to form the mosaic pattern of the elemental color phosphor dots. As a result, the following fact has been found.
FIG. 3 illustrates an exemplary pattern of landing on the mosaic pattern of the color emissive phosphor dots. In FIG. 3, reference numeral 25 represents a generally rectangular phosphor deposited screen having the mosaic pattern of the color emissive phosphor dots, and arrows identified by respective reference numerals 41 to 46 represent directions of displacement in landing characteristic (directions of mislanding) which were viewed with the use of a microscope. That is the direction in which the travel path of the electron beams relative to the mosaic pattern of the color emissive phosphor dots should be corrected in order for landing spots of the electron beams to strike upon the corresponding color emissive phosphor dots. Black dots identified by respective reference numerals 47 to 49 represent that the landing displacement is zero.
(a) As shown in FIG. 4, with reference to the relative landing characteristics at corner positions D of the luminescent phosphor deposited screen 25 and position E on the Y-axis thereof, the landing characteristics in the Y-axis direction are opposite to each other, and the difference thereof cannot be corrected. FIG. 3 illustrates that, in such a case, the landing displacement in the Y-axis direction is zeroed at the corner positions and, therefore, the landing displacement in the Y-axis direction appears in a considerable amount on the the Y-axis.
(b) As shown by the arrows 41, 43, 44 and 46 in FIG. 3, the landing displacement in the X-axis direction at the corners of the luminescent phosphor deposited screen 25 cannot be corrected.
In general, the landing displacement occurring at the corners of the luminescent phosphor deposited screen is considered problematic as compared with the landing displacement occurring at other positions of the same luminescent phosphor deposited screen. This is closely associated with the fact that the electron beams directed so as to impinge upon any one of the corner portions of the luminescent phosphor deposited screen are greatly deflected and are consequently apt to be adversely affected by an external magnetic field such as, for example, the terrestrial magnetic field.
Accordingly, it has long been desired that the landing displacement in the X-axis direction at the corner portions of the luminescent phosphor deposited screen such as indicated by the arrows 41, 43, 44 and 46 could be compensated for.